Chapter 6: Chemical Reactions and Equations

1
Chapter 6 Chemical Reactions and Equations

• Section 6.1 Chemical Equations

2

• Objectives Relate chemical changes and
  macroscopic properties, demonstrate how chemical
  equations describe chemical reactions, Illustrate
  how to balance chemical reactions by changing
  coefficients

3

• Identify the changes that take place when a cake
  is made

4
A Chemical Reaction is

• When a substance undergoes a chemical change
•  
• After a chemical reacts, it no longer has the
  same chemical identity

5
How Do You Recognize Chemical Change?

• A precipitate (solid) forms
• Heat is produced or absorbed
• Odor changes
• Gas is given off
• Color change and bubbles given off indicates a
  good chance a chemical change has occurred, but
  they are not proof positive.

6
Which of the following is a chemical change?

• growth of a tree
• melting butter
• the use of food in the body
• CO2 escaping from a can of soda
• separation of the components of crude oil through
  distillation
• a lake freezing

7

• Chemical Reaction (RXN) a process by which one
  or more substances are changed into one or more
  different substances represented by an equation.
• Chemical Equation (EQN) shows what change takes
  place, and shows the relative amounts of various
  elements/compounds that take part in these
  changes.

8
Reaction

• REACTANT Substance that undergoes a reaction
  (change)
• PRODUCT New substance formed
•  
• Example
• Iron and oxygen (reactants) ? iron (III) oxide
• (Fe3) (O2)
  (Fe2O3)
•  
• Reactions can involve single/ or many reactants
  or products

9
Word Equations

• Reactants are placed to the left of an arrow and
  products are placed to the right
•  
• Plus signs are used to separate reactants and
  also to separate products
• Example
• hydrochloric acid sodium hydroxide ? water
  sodium chloride
• (reactants) (products)

10
Chemical Equations

• Word equations can be converted into chemical
  equations
•  
• Chemical formulas replace the names of compounds
  or elements
•  

11
Chemical Equations

• Symbols in parentheses are put after formulas to
  indicate the state of the substance
• (s) solid
• (l) liquid
• (g) gas
• (aq) aqueous
• Example
• HCl(l) NaOH(s) ? H2O(l) NaCl(aq)

12
Energy and chemical equations

• Energy is often released or absorbed during
  chemical reactions
•  
• If energy is absorbed ? endothermic reaction
• If energy is released ? exothermic reaction
•  

13
Energy and chemical equations

• For reactions that absorb (need) energy, the word
  energy is often written along with the reactants
  in a chemical equation
• Example 2H2O(l) energy ? 2H2(g) O2(g)
•  

14
Energy and chemical equations

• For reactions where energy is released, the word
  energy is often written along with the products
  in a chemical equation
• Example
• CH4(g) 2O2(g) ? CO2(g) 2H20(g) energy

15
Energy and chemical equations

• Note The word energy is not always written. If
  it is important to know if energy is absorbed or
  released or in some cases where the reaction
  would not occur without the addition of energy

16
Conservation of mass

• The same amount of matter is contained in the
  products and reactants (matter is neither created
  nor destroyed)
• Mass of reactants Mass or products
• In chemical reactions, atoms do NOT change, they
  rearrange!

17
Conservation of atoms

• The number and kinds of atoms present in the
  reactants of the chemical reaction are the same
  as those present in the products
•  
• Counting atoms The subscript after the symbol
  for an element represents how many atoms of that
  element are present

18
Balancing Chemical Equations

• For a chemical equation to accurately represent a
  reaction, the same number of each kind of atom
  must be on the left side of the arrow as are on
  the right side ? It is said to be balanced

19
Balancing Chemical Equations

• To represent more than one unit taking part or
  being formed in a reaction, use a
• COEFFICIENT a number is placed in front to
  indicate the number of units involved
• To balance equations, coefficients can be added
  or changed on the reactants and products
• Subscripts CANT be changed changing subscripts
  changes the identity of that subs

20
Steps to write a balanced equation

1. Analyze
2. Set Up
3. Solve
4. Check

21
Balancing Chemical Equations

• 1. Magnesium metal and water combine to form
  solid magnesium hydroxide and hydrogen gas
•  
• Analyze (word equation)
• magnesium water ? magnesium hydroxide
  hydrogen
• Set up (chemical equation)
• Mg(s) H2O(l) ? Mg(OH)2(s) H2(g)
•  

22
Balancing Chemical Equations

• Solve (count atoms and then balance)
• Left Mg 1, H 2, O 1
• Right Mg 1, H 4, O 2
•  
• Mg(s) 2H2O(l) ? Mg(OH)2(s) H2(g)
•  

23

• Check (count atoms)
• Left Mg 1, H 4, O 2
• Right Mg 1, H 4, O 2
•  
• The equation is properly balanced.

24
Create and Balance

• Hydrogen and oxygen combine to form gaseous water
  and release energy.
• Word equation
• Hydrogen oxygen -gt water energy
• Chemical equation
• H2(g) O2(g) -gt H2O (g) energy
• Balance
• 2H2(g) O2(g) -gt 2H2O (g) energy

25
Practice Problems

• P. 199 (1,3)
• Write word equations and chemical equations
• 1) Magnesium and water combine to form solid
  magnesium hydroxide and hydrogen gas.
• 3) When energy is added to solid manganese(II)
  sulfate heptahydrate crystals, they break down to
  form liquid water and solid manganese(II) sulfate
  monohydrate

26
Practice Problems

• P. 199 (2,4)
• Write word equations and chemical equations
•  
• 2) An aq. Solution of dihydrogen dioxide and
  solid lead (II) sulfide combine to form solid
  lead (II) sulfate and liquid water.
•  
• 4) Solid potassium reacts with liquid water to
  produce aq. Potassium hydroxide and hydrogen gas